A hologram is a recording of an interference fringe pattern between two beams of light. One of these beams usually comprises light reflected from a real object and is called an "object beam," while the other beam is usually a pure and unmodulated beam and is called a "reference beam." If the object beam is pure and unmodulated like the reference beam, then a set of regular interference fringes is recorded and the hologram is referred to as a holographic diffraction grating. When a hologram is illuminated with only the reference beam, the object beam is exactly reproduced in phase and amplitude.
There are two types of holographic recording configurations that are in prevalent use today. The first is called an off-axis, or "Leith-Upatniaks," hologram. In forming this type of hologram, the object beam and the reference beam impinge upon a recording medium from the same side and from directions which are separated by a small angle. As shown in FIG. 1, the planes of the resulting interference fringes, which planes bisect the angle, are formed substantially perpendicular to the surface of the recording medium. Holographic diffraction gratings and embossed display holograms in use today are predominantly formed from this type of hologram.
This hologram is considered to be an improvement over the original hologram invented by Gabor, before the advent of the laser and its coherent light made off-axis configuration possible. With the Gabor, or in-line hologram, the object and reference beams fall in a line on the same axis. Because of the considerable difficulties involved in separating these beams, this hologram never came into wide use. As shall be seen later, however, one embodiment of the present invention exploits this configuration to useful advantage.
Embossed display holograms are typically formed from off-axis holograms in a multi-step process. The first step usually involves making a primary off-axis hologram where the real object is positioned some distance from the surface of the recording medium and the reference beam is a collimated or parallel beam. The second step usually involves illuminating the primary off-axis hologram with a collimated beam of light to project a real image of the object into space. A second hologram is then made by positioning a new recording medium at the position of the projected real image and by introducing a new reference beam at an angle. After development, the second hologram can be viewed under ordinary white light instead of laser light because color blurring is minimized for a focused image. Such a process is described in an article entitled "The Newport Button: The Large Scale Replication Of Combined Three- And Two-Dimensional Holographic Images," by J. J. Cowan, Proc. of SPIE, Vol. 462 Optics in Entertainment II, 1984, pp. 20-27.
In the second step described above, if the primary hologram is illuminated with a narrow slit of light instead of with a full aperture beam, the real image is brighter and deeper, but it exhibits parallax in the horizontal direction only and in a spectral hue of colors from blue to red in the vertical direction. The resulting "rainbow" hologram is the particular type of off-axis hologram which is most widely used today for embossed holograms.
In making embossed holograms, the recording medium used in the second step described above is typically photoresist. Photoresist is a material which, when developed, yields a surface profile whose depth is proportional to the intensity of the incident interference fringes. Since the intensity of the interference fringes of most holograms is essentially sinusoidal, the etch depth profile of the surface of a developed hologram resembles a sine wave when viewed on edge.
The third step of making an embossed hologram usually involves coating the surface of the photoresist of the hologram made in the second step with a conducting metal like silver and immersing the coated hologram in an electroplating bath to plate a layer, such as a layer of nickel, thereon. The fourth step involves using the nickel plate layer as a hard master to emboss the interference pattern into plastic that has been softened by heat, pressure, solvents or some combination thereof in a continuous fashion Finally, in the last step, after embossing, the plastic is typically coated with a highly reflecting metal, like aluminum, to enhance the diffraction efficiency of the embossed hologram.
The second type of hologram in prevalent use today is a volume phase reflection (VPR), or "Denisyuk," hologram. In forming this type of hologram, the object beam and the reference beam impinge upon the recording medium from opposite sides and from opposite directions. As shown in FIG. 2, the planes of the resulting interference fringes are formed substantially parallel to the surface of the recording medium. These planes are spaced apart within the recording medium at a distance which is equal to one-half the wavelength of the recording light divided by the index of refraction of the recording medium. Typical recording media used in the art are fine grained silver halide emulsions--for which the interference fringe planes comprise regions of high density of developed silver--or dichromated gelatin or photopolymer--for which the interference fringe planes comprise regions of slight differences in the index of refraction in comparison with lower exposed regions When a VPR hologram is illuminated with white light, only light having the same wavelength as that of the light that was used in recording is reflected back to the viewer. This occurs because the interference fringe planes that are stacked a half wavelength apart will only coherently backscatter light of that wavelength, i.e., they allow constructive interference. All other wavelengths destructively interfere and are scattered out of the field of view because they do not match the spacing of the planes.
A VPR hologram advantageously allows viewing at full parallax and in a single color, rather than the whole spectrum of colors which is characteristic of an off-axis hologram. Further, high diffraction efficiencies can be achieved without requiring a reflective metallic coating. Still further, a VPR can be viewed from almost any angle of illumination, whereas an off-axis hologram can usually be viewed only with overhead illumination.
Notwithstanding the above-described advantages of a VPR hologram, it has not generally been considered to be feasible to replicate this structure by embossing because the fringe structure comprises a stack of parallel planes disposed within the body of the recording medium. This occurs because development of a photoresist necessarily stops at the first layer. Consequently, VPR holograms are presently replicated by optical, rather than mechanical means, and only then by using a laser. The presently known, and conventional, method of replicating VPR holograms comprises the following steps: (1) making a master hologram recording plate by directing an object beam and a reference beam to impinge upon a first recording medium from opposite sides; and (2) placing a second recording plate in front of the developed master plate and passing a second reference beam therethrough--the object light from the master is reflected and passes in the opposite direction to the second reference beam in the second recording plate and a new VPR hologram will be recorded in the new recording plate. Conventional VPR holograms can be mass replicated.
As a result, there is a need in the art for a volume phase reflection hologram which can be easily replicated.